This is caused by the fact that the mind estimates the distance of an object partly by comparing the two views of its surface obtained by the two eyes respectively, and which views are not quite the same, but differ, just as the two pictures prepared for the stereoscope. Each eye sees a little round one side of the object, and the solid look of the object and its distance are thus before the mind.
Now when the rope is horizontal the eyes do not see round the two sides in this manner, though if the head is leant sideways it will be found that the illusion referred to no longer appears.
Nor is it out of place to inquire thus at length into this matter, for I can assure you that one or two blunt slaps on the head from these ropes across a river make it at least interesting if not pleasant to examine "the reason why." And now we have got the philosophy of the thing, let us leave the ropes behind.
The actual number of miles in a day's work is much influenced by the number of waterfalls or artificial barriers which are too dry or too high to allow the canoe to float over them.
"Shirking a Fall."
In all such cases, of course, I had to get out and to drag the boat round by the fields, as has been already described (p. 80); or to lower her carefully among the rocks, as is shown in the accompanying sketch, which represents the usual appearance of this part of the day's proceedings. Although this sort of work was a change of posture, and brought into play new muscular action, yet the strain sometimes put on the limbs by the weight of the boat, and the great caution required where there was only slippery footing, made these barriers to be regarded on the whole as bores.
Full soon however we were to forget such trifling troubles, for more serious work impended.
The river banks suddenly assumed a new character. They were steep and high, and their height increased as we advanced between the two upright walls of stratified gravel and boulders.
A full body of water ran here, the current being of only ordinary force at its edges, where it was interrupted by rocks, stones, and shingle, and was thus twisted into eddies innumerable.